I. Field of the Invention
The present invention pertains to active, multi-layered, positive electrode materials. More particularly, the present invention pertains to a multi-layered, nickel hydroxide particle and a method for making the same.
II. Description of the Prior Art
There presently exists a need for a nickel hydroxide powder having enhanced intra-particle properties such as cycle life, conductivity and improved high temperature performance without reduced capacity, the invention of which has not been addressed heretofore or met by the below discussed references.
Nickel hydroxide is used as an active positive electrode material in several types of battery systems and has been commercially available for a number of years. Two types of battery systems include the highly toxic Nixe2x80x94Cd (nickel cadmium) type and the more desirable Nixe2x80x94MH (nickel metal hydride) type.
Nixe2x80x94MH cells utilize a negative electrode that is capable of reversible electrochemical hydrogen storage. Nixe2x80x94MH cells employ a positive electrode made with nickel hydroxide material. The negative and positive electrodes are spaced apart in an alkaline electrolyte. Upon application of an electrical potential across a Nixe2x80x94MH cell, the Nixe2x80x94MH material of the negative electrode is charged by the electrochemical absorption of hydrogen and the electrochemical discharge of an hydroxyl ion. The negative electrode reactions are reversible, as shown in equation 1.
M+H2O+exe2x88x92⇄Mxe2x88x92H+OHxe2x88x92xe2x80x83xe2x80x83(1)
Upon discharge, the stored hydrogen is released to form a water molecule and release an electron.
The reactions that take place at the nickel hydroxide positive electrode are shown in equation 2.
Ni(OH)2+OHxe2x88x92⇄NiOOH+H2O+exe2x88x92xe2x80x83xe2x80x83(2)
The use of nickel hydroxide as a positive electrode material for batteries is generally known. See for example, U.S. Pat. No. 5,523,182, issued Jun. 4, 1996 to Ovshinsky et al., entitled xe2x80x9cEnhanced Nickel Hydroxide Positive Electrode Materials For Alkaline Rechargeable Electrochemical Cellsxe2x80x9d, the disclosure which is herein incorporated by reference.
Nickel metal hydride batteries are typically positive material limited or negative material biased. This means that more active negative material is present in a cell than active positive material. The purpose of negatively biasing a battery is to prevent damage to the active negative material during over-charging. Over-charging a non-biased nickel metal hydride battery produces competing reactions which can oxidize and corrode negative metals. The resulting corrosion is not reversible, therefore over charging can permanently damage active negative material. Because the cell is negative material biased, the capacity of a cell is determined by the capacity of the active positive material present. The active positive material adds a significant amount of weight to a battery. By improving the capacity of the active positive material, less positive material is required per cell, thereby reducing the overall battery weight.
Several forms of positive electrodes exist at the present and include sintered, foamed, and pasted electrode types. Processes for making positive electrodes are generally known in the art, see for example U.S. Pat. No. 5,344,728 issued to Ovshinsky et al., the disclosure of which is herein incorporated by reference, where capacity in excess of 560 mAh/cc was reported. The particular process used can have a significant impact on an electrode""s performance. For example, conventional sintered electrodes normally have an energy density of around 480-500 mAh/cc. Sintered positive electrodes are constructed by applying nickel powder slurry to a nickel-plated, steel base followed by sintering at high temperature. This process causes the individual particles of nickel to weld at their points of contact, resulting in a porous material that is approximately 80% open volume and 20% solid metal. This sintered material is then impregnated with active material by soaking it in an acidic solution of nickel nitrate, followed by the conversion to nickel hydroxide by reaction with an alkali metal hydroxide. After impregnation, the material is subjected to electrochemical formation.
To achieve significantly higher loading, the current trend has been away from sintered positive electrodes and toward pasted electrodes. Pasted electrodes consist of nickel hydroxide particles in contact with a conductive network or substrate, most commonly nickel foam. Several variants of these electrodes exist and include plastic-bonded nickel electrodes, which may utilize graphite as a microconductor, and pasted nickel fiber electrodes, which utilize spherical nickel hydroxide particles loaded onto a high porosity, conductive nickel fiber or nickel foam support.
As with electrode formation, the properties of nickel hydroxide also differ widely depending upon the production method used. Generally, nickel hydroxide is produced using a precipitation method in which a nickel salt solution and a hydroxide salt solution are mixed together followed by the precipitation of nickel hydroxide. Active, nickel hydroxide material preferably has high capacity and long cycle life, see U.S. Pat. No. 5,348,822 to Ovshinsky et al., the disclosure of which is herein incorporated by reference.
In order to produce high density, substantially spherical particles, nickel hydroxide crystals are grown relatively gradually under carefully controlled process conditions. A nickel salt provided in solution is combined with an ammonium ion. The nickel salt forms complex ions with ammonia to which caustic is added. Nickel hydroxide is then gradually precipitated by decomposition of the nickel ammonium complex. The reaction rate is difficult to control, so methods have been introduced to separate critical reaction steps in the production process to compensate for said difficulties. For example, U.S. Pat. No. 5,498,403, entitled xe2x80x9cMethod for Preparing High Density Nickel Hydroxide Used for Alkali Rechargeable Batteriesxe2x80x9d, issued to Shin on Mar. 12, 1996, the disclosure of which is herein incorporated by reference, discloses a method of preparing nickel hydroxide from a nickel sulfate solution using a separate or isolated amine reactor. Nickel sulfate is mixed with ammonium hydroxide in the isolated amine reactor to form nickel ammonium complex. The nickel ammonium complex is removed from the reactor and sent to a second mixing vessel or reactor where it is combined with a solution of sodium hydroxide to obtain nickel hydroxide. Such a method relies heavily on a raw material source of very high purity.
Improvements to nickel hydroxide materials have received significant attention in recent years as can be seen from the following patents.
U.S. Pat. No. 5,861,225 entitled xe2x80x9cNickel Battery Electrode Having Multiple Composite Nickel Hydroxide Active Materials,xe2x80x9d issued Jan. 19, 1999 to Corrigan et al. discloses a nickel hydroxide material having discrete layers of differing redox potentials to improve active material utilization by forcing a stepwise discharge of the material to avoid isolating active material regions.
U.S. Pat. No. 5,523,182 entitled xe2x80x9cEnhanced Nickel Hydroxide Positive Electrode Materials For Alkaline Rechargeable Electrochemical Cells,xe2x80x9d issued Jun. 4, 1996 to Ovshinsky et al. discloses a cobalt hydroxide layer or cobalt oxyhydroxide encapsulant layer formed about a positive electrode material. The patent teaches the desirability of an outer coating rich in cobalt.
U.S. Pat. No. 5,840,269 to Shin et al discloses a double layer nickel hydroxide material having a thin outer shell with high cobalt concentrations. The spray procedure provides a thin layer of material with over 10 wt % cobalt and which is less than 6% of the total mass. The process conditions utilized in Shin, being 2.0-2.8 mole/liter NiSO4, ammonium hydroxide at 12-16 mole/liter, and NaOH at 5-8 mole/liter are inappropriate for a practical commercial process, and generate excess amounts of waste while making achievement of sufficient tap density practically impossible. While it may be generally known to have a thin outer layer of active material with high cobalt, the art has not gone further in teaching or suggesting formulations that offer improvements over such disclosures. Thus, Shin does little to suggest any benefits nor does it advance the art as related to increasing material performance, such as cycle life or capacity, other than by adjusting the outer layer for a thin but high cobalt concentration, which was previously taught in U.S. Pat. No. 5,523,182 described above, and assigned to the assignee of the present invention. Further, Shin does not suggest any benefit to using more than a two element outer layer. Additionally, the Shin process is not suitable for depositing Ca in the outer layer due to premature precipitation problems with the Shin preamine reactor system. Therefore, Shin does not provide an enabling disclosure as to Ca.
U.S. Pat. No. 5,611,823 entitled xe2x80x9cMethod for Fabricating a Battery Electrode,xe2x80x9d discloses an electrochemically active material prepared by coating non-conductive to low-conductive particles of a metal hydroxide or metal oxide powder with nickel to enhance conductivity of the particles using an initial sensitizing solution.
As can be seen in the above disclosures, a thin external layer has been added to nickel hydroxide particles to improve particle-to-particle interaction. But, even in this highly crowded field of active positive materials, modern day nickel hydroxide materials have failed to deliver the requisite intra-particle, high storage capacity, high temperature performance, high rate, low cost, long cycle life materials needed for further advancement of rechargeable batteries.
The present invention, to address the above mentioned deficiencies and others, provides a new and improved active positive electrode particle having a plurality of active materials. The active materials include a first active positive electrode material; and a second active positive electrode material disposed about the first material, the second active positive electrode material having a mass that is 10% or greater than the total particle mass.
In a preferred aspect hereof the second material has a plurality of channels extending from an outer most portion to an inner-most portion of the second material for providing ion transport between an electrolyte and the first active material.
In preferred embodiment hereof, the active positive electrode particle is a nickel hydroxide particle having a first active nickel hydroxide material, and a second active nickel hydroxide material disposed about the first material, wherein the second active nickel hydroxide material has a mass of at least 10% of the particle mass.
In another preferred embodiment of the present invention the first material and the second material have different but similar compositions with a compositional difference of 1 to 25 atomic % and more preferably 1 to 18 atomic %.
In a third preferred embodiment hereof there is provided a positive electrode particulate as described above having enhanced conductivity, wherein the first and/or the second active material or both have an embedded metallic conductive material.
In a fourth preferred embodiment hereof there is provided a multi-compositional positive electrode particulate as described above wherein the second material is an outer material having a cycle life greater than the first or inner material.
In another aspect of the present invention there is provided a method of making a multi-compositional nickel hydroxide material by exposing an active seed material to a precipitation reaction for a residence time sufficient to provide a layer of active material having a mass of at least 10% of the particle mass on the seed.
Particles of the present invention may be combined to form a powder for use in electrochemical battery electrodes. For a more complete understanding of the present invention, reference is now made to the following detailed description of preferred embodiments and the accompanying drawings.